1. Field of the Invention
The present invention relates to a golf ball, and more particularly, to a golf ball having improved aerodynamic characteristics through a combination of dimple factors including the total number of dimples on the surface of the golf ball, the ratio of the total surface area of the golf ball to the total surface areas of the dimples, and the volumes of the respective dimples corresponding to the regions or zones of the golf ball, etc.
2. Description of Related Arts
Normally, on the surface of a golf ball, 300 to 550 dimples in number are provided, the principal role of which is to improve aerodynamic characteristics of the golf ball, to optimize the trajectory, and to increase the carry or flying distance thereof.
There are many dimple factors that affect the trajectory of the golf ball. One of the factors which has a large influence on the trajectory the ratio of the total area of the dimples to the surface area of the golf ball (i.e. surface area occupying rate by the dimples).
Conventionally, studies have been made with respect to the surface area occupying rate of the dimples, and various techniques, such as those listed below, have been proposed.
(1) Japanese patent publication Tokkosho No. 58-50744 discloses a golf ball in which dimples are closely arranged through a reduction in pitch or by making the interval therebetween as small as possible.
(2) Japanese patent laid-open publication Tokkaisho No. 62-192181 discloses an arrangement in which, by utilizing dimples of differing diameters, the dimples are so closely arranged that in one land portion (i.e. the surface portion without any dimple) surrounded by the dimples, a new dimple having an area larger than an average area of the plurality of dimple sizes can not be formed.
(3) In Japanese patent laid-open publication Tokkaisho No. 64-8982, the surface area occupying rate of the dimples is over 65% through the use of non-circular dimples.
(4) In Japanese patent laid-open publication Tokkaisho No. 63-309282, the surface area occupying rate of the dimples is set to be more than 78% through a combination of two kinds of large and small dimples.
The known techniques listed above have a common technical concept. Namely, in that the dimples provided on the surface of the golf ball are arranged as close to each other as possible in order to improve the surface area occupying rate of the dimples, and thereby improve the aerodynamic characteristics and carry of the golf ball.
However, it is also known that the aerodynamic characteristics of the golf ball are largely affected by the total number of dimples, and in order to improve the aerodynamic characteristics of the golf ball for proper trajectory and increased carry, it is originally required to collectively take into account, both the surface area occupying rate of the dimples and the total number of dimples.
Conventionally, the above two factors are considered to be independent of each other, and in the prior art technique with respect to the surface area occupying rate of the dimples described above, the relation thereof with respect to the total number of dimples has not been particularly referred to. In other words, such prior art technique is based on a concept that the flight performance is improved through improvement of the surface area occupying rate of the dimples, irrespective of the total number of dimples, and thus, does not collectively take into consideration both the dimple surface area occupying rate and the total number of dimples.
Meanwhile, since the golf ball as described above is normally molded by a split metallic mold composed of semi-spherical upper and lower molds, a burr is formed at a junction of the molds, i.e. at a parting line between the upper and lower molds during the molding. Such a burr is scraped off in a later processing step by buffing wherein a seam portion is formed. Accordingly, dimples can not be provided on the seam portion so as to allow buffing of the burr. In other words, on the seam portion at both sides of the parting line, a great circle zone is formed which does not intersect any dimples and thus presents an aerodynamic characteristic that is clearly different from that of the spherical surfaces of other golf balls.
More specifically, when the golf ball is struck so that a line connecting both poles becomes a rotary axis of "back-spin", the portion where the circumferential speed of the rotary axis is the fastest coincides with the seam portion, thus undesirably lowering the trajectory height as compared with that when another line is set to be the rotary axis. Normally, the dimples at the portion where the circumferential speed is the fastest affect the trajectory height by the largest extent. Thus, with the seam portion, where no dimples are formed, at the fastest circumferential speed, the dimple effect of the golf ball on the whole is decreased and the generation of the lift is reduced. As described above, when the golf ball is hit, the trajectory height is undesirably altered depending on the portions struck by a club. Thus, not only is the interest of golf as a game reduced, but such a golf ball having a poor symmetrical nature is to be against the official rules set by USGA (United States Golf Association).
In order to solve the problem as described above, there has been proposed, for example, in Japanese Patent Laid-Open Publication Tokkaisho No. 61-284264, a technique in which by making the dimple volume in the vicinity of the seam portion larger than that in the vicinity of the pole, the dimple effect around the seam portion is improved, and thus the symmetrical nature of the aerodynamic characteristic of the golf ball on the whole is improved.
However, in the prior art technique as referred to above, although the ratio of the dimple volume in the vicinity of the seam portion, to the dimple volume in the vicinity of the pole is not stated from the viewpoint of the relation with respect to the surface area occupying rate of the dimples, it should be noted that these are closely related to each other. More specifically, as the dimples are arranged more closely at a larger surface area occupying rate, the difference in the dimple effect between the seam portion without any dimples formed and the other portion on the surface of the golf ball becomes larger. Accordingly, it is desirable to considerably increase the ratio of the dimple volume in the vicinity of the pole, to the dimple volume in the vicinity of the seam portion. On the other hand, when the surface area occupying rate of the dimples is small, i.e. when the dimples are arranged sparsely, the difference in the dimple effect between the seam portion without any dimple and the other portion of the surface of the golf ball becomes small. Accordingly, it is sufficient to slightly increase the ratio of the dimple volume in the vicinity of the seam portion, to the dimple volume in the vicinity of the pole.